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Sans is a minor character in the SMG4 series.
Sans shows up in Snowdin along with his brother Papyrus. When Papyrus captures Mario (and falls in love with him) Sans leaves the house telling them "not to make too many babies", as a joke. After Mario destroys the police and the Giant dancing Papyrus robot, Sans shows up saying, "What monster could have done this?". He complains to Papyrus for letting the "human" escape. Mario faces Sans at the end where they have a big fight, like in the original Undertale. In the end, though, Mario pokes sans in his "sexy" eye, leaving him shaking on the ground in pain.
Since his debut, he has appeared in many ways as cameos as a joke.
In "Sans's First Day in Smash", he plays a big role in the episode. He appears along Mario, Link, Banjo & Kazooie on a smash stage, but he sits aside and sleeps while the others fight. Mario takes notice of this and confronts Sans on their last encounter. Sans reveals that he no longer holds any hostility towards the plumber and throws him off the stage as he follows behind, thus declaring Banjo the winner after killing Link. He appears in the lobby where he's sitting on the couch and has a slouching contest with Mario until they end up on the floor. Sans and Mario both decide to leave, until being confronted by Master Hand. Sans shows no signs of fear and endures all of his attacks until Papyrus arrives. There, he showcases his powers and defeats the manacle glove and drives off with his brother.
He again makes a return as a major character in "Steve vs Smash Bros", acting as Master Hand's buddy for most of the episode as Steve fights many fighters. He eventually fights Steve himself, but stops once he acknowledges his power. Master Hand, however, wants the two to kill each other. Once Mario, Steve and Sans band together, they defeat him once more.